Chronic estrogen depletion alters adenosine diphosphate-induced pial arteriolar dilation in female rats

Effect of estradiol on enzymes of vascular extracellular nucleotide metabolism

PURPOSE

Estrogens have beneficial effects on the cardiovascular system, promoting vasodilation, endothelial cells growth, relaxation, and regulation of blood pressure. Some of these effects could be associated with the purinergic system known for the control of vasodilation, inflammation, and platelet function. The aim of our study was the evaluation of ATP, AMP, and adenosine extracellular catabolism, catalyzed by ectonucleoside triphosphate diphosphohydrolase-1 (CD39), ecto-5'-nucleotidase (CD73), and ecto-adenosine deaminase (eADA) in mouse aortas.

METHODS

Extracellular hydrolysis of ATP, AMP, and adenosine was estimated on the aortic surface of 3-month-old female and male C57BL/6 J wild-type (WT) mice, in female WT mouse aortas incubated for 48 h in the presence or absence of 100 nM estradiol, and in WT female mouse and ApoE-/-LDL-R-/- aortas. The conversion of substrates to products was analyzed by high-pressure liquid chromatography (HPLC).

RESULTS

We demonstrated significantly higher adenosine deamination rate in WT male vs. female mice (p = 0.041). We also noted the lower adenosine hydrolysis in aortas exposed to estradiol, as compared with the samples incubated in estradiol-free medium (p = 0.043). Finally, we observed that adenosine conversion to inosine was significantly higher on the surface of ApoE-/-LDL-R-/- aortas compared with WT mice (p = 0.001). No such effects were noted in ATP and AMP extracellular hydrolysis.

CONCLUSION

We conclude that estradiol inhibits the extracellular degradation of adenosine to inosine, which may be an element of its vascular protective effect, as it will lead to an increase in extracellular adenosine concentration. We can also assume that during the development of the atherosclerotic process, the protective role of estradiol in the regulation of adenosine degradation may be obscured by other pathogenic factors.

Sex differences in the vascular function and related mechanisms: role of 17β-estradiol

The incidence of cardiovascular disease (CVD) is lower in premenopausal women but increases with age and menopause compared with similarly aged men. Based on the prevalence of CVD in postmenopausal women, sex hormone-dependent mechanisms have been postulated to be the primary factors responsible for the protection from CVD in premenopausal women. Recent Women’s Health Initiative studies, Cochrane Review studies, the Early Versus Late Intervention Trial with Estradiol Study, and the Kronos Early Estrogen Prevention Study have suggested that beneficial effects of hormone replacement therapy (HRT) are seen in women of <60 yr of age and if initiated within <10 yr of menopause. In contrast, the beneficial effects of HRT are not seen in women of >60 yr of age and if commenced after 10 yr of menopause. The higher incidence of CVD and the failure of HRT in postmenopausal aged women could be partly associated with fundamental differences in the vascular structure and function between men and women and in between pre- and postmenopausal women, respectively. In this regard, previous studies from human and animal studies have identified several sex differences in vascular function and associated mechanisms. The female sex hormone 17β-estradiol regulates the majority of these mechanisms. In this review, we summarize the sex differences in vascular structure, myogenic properties, endothelium-dependent and -independent mechanisms, and the role of 17β-estradiol in the regulation of vascular function.

Purinergic mechanisms in gliovascular coupling

Regional elevations in cerebral blood flow (CBF) often occur in response to localized increases in cerebral neuronal activity. An ever expanding literature has linked this neurovascular coupling process to specific signaling pathways involving neuronal synapses, astrocytes and cerebral arteries and arterioles. Collectively, these structures are termed the "neurovascular unit" (NVU). Astrocytes are thought to be the cornerstone of the NVU. Thus, not only do astrocytes "detect" increased synaptic activity, they can transmit that information to proximal and remote astrocytic sites often through a Ca(2+)- and ATP-related signaling process. At the vascular end of the NVU, a Ca(2+)-dependent formation and release of vasodilators, or substances linked to vasodilation, can occur. The latter category includes ATP, which upon its appearance in the extracellular compartment, can be rapidly converted to the potent vasodilator, adenosine, via the action of ecto-nucleotidases. In the present review, we give consideration to experimental model-specific variations in purinergic influences on gliovascular signaling mechanisms, focusing on the cerebral cortex. In that discussion, we compare findings obtained using in vitro (rodent brain slice) models and multiple in vivo models (2-photon imaging; somatosensory stimulation-evoked cortical hyperemia; and sciatic nerve stimulation-evoked pial arteriolar dilation). Additional attention is given to the importance of upstream (remote) vasodilation; the key role played by extracellular ATP hydrolysis (via ecto-nucleotidases) in gliovascular coupling; and interactions among multiple signaling pathways.

Effects of estrogen on postischemic pial artery reactivity to ADP

OBJECTIVE

The aims of this work were to determine if 1) ischemia alters pial artery responsiveness to the partially nitric oxide (NO)-dependent dilator, ADP, 2) the alteration depends on 17beta-estradial (E2), and 3) NO contributes to E2 protective effects.

MATERIALS AND METHODS

Response to ADP and the non-NO-dependent dilator, PGE(2), were examined through closed cranial windows. Ovariectomized (OVX) and E2-replaced (E25, 0.025 mg; or E50, 0.05 mg) rats were subjected to 15-minute forebrain ischemia and one-hour reperfusion. Endothelial NO synthase (eNOS) expression was determined in pre- and postischemic isolated cortical microvessels.

RESULTS

In OVX rats, ischemia depressed pial responses to ADP, but not to PGE(2). Both doses of E2 maintained responses to ADP and had no effect on the response to PGE(2). eNOS inhibition decreased the ADP response by 60% in the E25 rats and 50% in the E50 rats, but had no effect in the OVX rats. Compared to the OVX group, microvessel expression of eNOS was increased by E2, but postischemic eNOS was unchanged in both groups.

CONCLUSIONS

The nearly complete loss of postischemic dilation to ADP suggests that normal non-NO-mediated dilatory mechanisms may be acutely impaired after ischemic injury. Estrogen's protective action on ADP dilation may involve both NO- and non-NO-mediated mechanisms.

Effects of adenine on the pituitary-gonad axis in newborns rats

The present study was undertaken to investigate effects of the adenine on the hypothalamus-pituitary-gonad axis and changes in blood hormone concentration such as FSH, LH, progesterone and estrogen in newborn female rats. Adenine is a common organic base and its concentration variations caused by foods, has various effects on the body metabolic systems. In present study, fifty newborns rats were used divided into five groups, of 10s, including control I, control II which received solvent (normal saline) only and three experimental groups which received 50, 100 and 200 mg body weight adenine respectively. All the animals were kept under same condition with plenty food and water and treated Intra Peritoneally (IP) during days 2-16 after birth. At the end of experiment, all the animals were weighed, their ovaries were removed and blood samples were taken for hormone analysis. The results showed that dose dependent adenine solution significantly reduced the body and ovarian weight on 30 and 70 days after birth. In addition adenine led into no significant difference in concentration of FSH and LH in the experimental groups relative to the control on 30th day of life. But on the 70th day, the levels of these hormones raised significantly in the experimental groups. Furthermore, the adenine solution significantly increased the levels of progesterone and estrogen hormones in the experimental groups relative to the control on the 30th day, while decreased their concentration significantly on the 70th day. This situation has close similarities to metabolic disorders present in human caused by excessive use of adenine. High amounts consumption of adenine in can lead into hormone abnormality, weight loss and metabolic anomalies.

CEREBROVASCULAR EFFECTS OF OESTROGEN: MULTIPLICITY OF ACTION

1. Cerebral vessels express oestrogen receptors (ER) in both the smooth muscle and endothelial cell layers of cerebral blood vessels. Levels of ERalpha are higher in female rats chronically exposed to oestrogen, either endogenous or exogenous. 2. Chronic exposure to oestrogen, either endogenous (normally cycling females) or exogenous (ovariectomized with oestrogen replacement), results in cerebral arteries that are more dilated than arteries from ovariectomized counterparts when studied in vitro. This effect is primarily mediated by an increase in the production of vasodilator factors, including nitric oxide (NO) and prostacylin. In contrast, oestrogen appears to suppress the production of endothelial-derived hyperpolarizing factor. Oestrogen treatment increases cerebrovascular levels of endothelial nitric oxide synthase (eNOS), cyclo-oxygenase (COX)-1 and prostacyclin synthase. In addition, via activation of the phosphatidylinositol 3-kinase/Akt pathway, both acute and chronic oestrogen exposure increases eNOS phosphorylation, increasing NO production. 3. Oestrogen receptors have also been localized to cerebrovascular mitochondria and exposure to oestrogen increases the efficiency of energy production while simultaneously reducing mitochondrial production of reactive oxygen species. Oestrogen increases the production of mitochondrial proteins encoded by both mitochondrial and nuclear DNA, including cytochrome c, subunits I and IV of complex IV and Mn-superoxide dismutase. Oestrogen treatment increases the activity of citrate synthase and complex IV and decreases mitochondrial production of H(2)O(2). 4. Oestrogen also has potent anti-inflammatory effects in the cerebral circulation that may have important implications for the incidence and severity of cerebrovascular disease. Administration of lipopolysaccharide or interleukin-1beta to ovariectomized female rats induces cerebrovascular COX-2 and inducible nitric oxide synthase (iNOS) protein expression and increases prostaglandin E(2) expression. Levels of COX-2 and iNOS expression vary with the stage of the oestrous cycle, and the cerebrovascular inflammatory response is suppressed in ovariectomized animals treated with oestrogen. Interleukin-1beta induction of COX-2 protein is prevented by treatment with a nuclear factor (NF)-kappaB inhibitor, and oestrogen treatment reduces cerebrovascular NF-kappaB activity. 5. Cerebrovascular dysfunction and pathology contribute to the pathogenesis of stroke, brain trauma, oedema and dementias, such as Alzheimer's disease. A better understanding of the action of oestrogen on cerebrovascular function holds promise for the development of new therapeutic entities that could be useful in preventing or treating a wide variety of cerebrovascular diseases.

ATP release and hydrolysis contribute to rat pial arteriolar dilatation elicited by neuronal activation

Owing to their intimate anatomical relationship with cerebral arterioles, astrocytes have been postulated as signal transducers, transferring information from activated neurones to the cerebral microcirculation. These forwarded signals may involve the release of vasoactive factors from the end-feet of astrocytes. This mechanism is termed 'neurovascular coupling' and its anatomical components (i.e. neurone, astrocyte and vascular cells) are termed the 'neurovascular unit'. The process of neurovascular coupling often involves upstream dilatation. This is necessary during periods of increased metabolic demand, in order to permit more blood to reach dilated downstream vessels, thereby improving nutrient supply to the activated neurones. Without it, that downstream dilatation might be ineffective, placing neurones at risk, especially during episodes of intense neuronal activity, such as seizure. In the brain, pial arterioles represent important 'upstream' vascular segments. The pial arterioles overlie a thick layer of astrocytic processes, termed the glia limitans. This essentially isolates pial arterioles, anatomically, from the neurones below. Vasodilating signals that originate in the neurones therefore reach the pial arterioles via indirect pathways, primarily involving astrocytes and the glia limitans. Here we discuss a process whereby purinergic mechanisms play a key and neuronal activity-dependent role in astrocyte to astrocyte communication, as well as in glia limitans to pial arteriolar signals leading to vasodilatation.

DCEBIO-Mediated Dilations Are Attenuated in the Female Rat Middle Cerebral Artery

BACKGROUND

Unlike in peripheral vessels, the endothelium-derived hyperpolarizing factor (EDHF)-mediated component to P2Y(2) receptor-mediated dilations is significantly attenuated in the middle cerebral artery (MCA) of female rats compared to male rats. One aspect to the EDHF phenomenon is activation of the intermediate calcium-sensitive potassium (IK(Ca)) channels located on the endothelium. In an attempt to pinpoint the site along the EDHF pathway that is compromised in females, we tested the hypothesis that direct activation of IK(Ca) channels with DCEBIO would elicit attenuated hyperpolarization in the endothelium and smooth muscle of females compared to males.

METHODS

Inhibitors of nitric oxide synthase and cyclooxygenase were present throughout all experiments. Vessel diameter changes were assessed in pressurized and luminally perfused MCAs. Membrane potential changes in the endothelium and smooth muscle were measured using the perforated patch clamp method and sharp electrodes, respectively.

RESULTS AND CONCLUSIONS

The maximum vasodilation to 3 x 10(-4)M DCEBIO was significantly reduced in females (37 +/- 9%) compared to intact males (70 +/- 4%). Endothelial cell hyperpolarization to DCEBIO was similar in both males and females. Smooth muscle cell hyperpolarization was attenuated in females (2 +/- 1 mV) compared to males (15 +/- 3 mV). Taken together, our data suggest that the transfer of hyperpolarization from the endothelium to the smooth muscle is impeded in the female rat MCA.

Impaired cAMP signaling does not account for the attenuated EDHF-mediated dilations in female rat middle cerebral artery

Dilations to endothelium-derived hyperpolarizing factor (EDHF) are significantly attenuated in the middle cerebral artery (MCA) isolated from female compared to male rats. Since gap junctions appear to be involved in the EDHF pathway and cAMP has been shown to enhance gap junction permeability, we tested the hypothesis that elevation of cAMP would enhance EDHF-mediated dilations in female rat MCA. Vascular diameter was measured in perfused MCA segments using videomicroscopy in the presence and absence of IBMX, an inhibitor of cAMP phosphodiesterase. In the presence of L-NAME and indomethacin, dilation to 10(-4) M ATP was significantly reduced in females (48+/-12%) compared to males (92+/-2%). IBMX, an inhibitor of cAMP phosphodiesterase, had no significant effect on ATP-mediated dilations in both males and females. Basal cAMP levels were comparable in male and female MCAs (1.7 pmol/mg protein). Incubation with IBMX (2 x 10(-4) M) significantly elevated cAMP in both male (12.8 pmol/mg protein) and female (11.2 pmol/mg protein) MCAs. Our results demonstrate that reduced EDHF dilations in female rat MCA cannot be solely attributed to impaired cAMP signaling. Future studies will target other potential sites along the EDHF pathway in order to identify why EDHF dilations are reduced in the female compared to the male rat MCA.

Influence of sex steroid hormones on cerebrovascular function

The cerebral vasculature is a target tissue for sex steroid hormones. Estrogens, androgens, and progestins all influence the function and pathophysiology of the cerebral circulation. Estrogen decreases cerebral vascular tone and increases cerebral blood flow by enhancing endothelial-derived nitric oxide and prostacyclin pathways. Testosterone has opposite effects, increasing cerebral artery tone. Cerebrovascular inflammation is suppressed by estrogen but increased by testosterone and progesterone. Evidence suggests that sex steroids also modulate blood-brain barrier permeability. Estrogen has important protective effects on cerebral endothelial cells by increasing mitochondrial efficiency, decreasing free radical production, promoting cell survival, and stimulating angiogenesis. Although much has been learned regarding hormonal effects on brain blood vessels, most studies involve young, healthy animals. It is becoming apparent that hormonal effects may be modified by aging or disease states such as diabetes. Furthermore, effects of testosterone are complicated because this steroid is also converted to estrogen, systemically and possibly within the vessels themselves. Elucidating the impact of sex steroids on the cerebral vasculature is important for understanding male-female differences in stroke and conditions such as menstrual migraine and preeclampsia-related cerebral edema in pregnancy. Cerebrovascular effects of sex steroids also need to be considered in untangling current controversies regarding consequences of hormone replacement therapies and steroid abuse.

Novel aspects of endothelium-dependent regulation of vascular tone

The vascular endothelium plays a crucial role in the regulation of vascular homeostasis and in preventing the initiation and progress of cardiovascular disease by controlling mechanical functions of the underlying vascular smooth muscle. Three vasodilators: nitric oxide (NO), prostacyclin, and endothelium-derived hyperpolarizing factor, produced by the endothelium, underlie this activity. These substances act in a co-ordinated interactive manner to maintain normal endothelial function and operate as support mechanisms when one pathway malfunctions. In this review, we discuss recent advances in our understanding of how gender influences the interaction of these factors resulting in the vascular protective effects seen in pre-menopausal women. We also discuss how endothelial NO synthase (NOS) can act in both a pro- and anti-inflammatory action and therefore is likely to be pivotal in the initiation and time course of an inflammatory response, particularly with respect to inflammatory cardiovascular disorders. Finally, we review recent evidence demonstrating that it is not solely NOS-derived NO that mediates many of the beneficial effects of the endothelium, in particular, nitrite acts as a store of NO released during pathological episodes associated with NOS inactivity (ischemia/hypoxia). Each of these more recent findings has emphasized new pathways involved in endothelial biology, and following further research and understanding of the significance and mechanisms of these systems, it is likely that new and improved treatments for cardiovascular disease will result.

Endothelial influences on cerebrovascular tone

The cerebrovascular endothelium exerts a profound influence on cerebral vessels and cerebral blood flow. This review summarizes current knowledge of various dilator and constrictor mechanisms intrinsic to the cerebrovascular endothelium. The endothelium contributes to the resting tone of cerebral arteries and arterioles by tonically releasing nitric oxide (NO•). Dilations can occur by stimulated release of NO•, endothelium-derived hyperpolarization factor, or prostanoids. During pathological conditions, the dilator influence of the endothelium can turn to that of constriction by a variety of mechanisms, including decreased NO• bioavailability and release of endothelin-1. The endothelium may participate in neurovascular coupling by conducting local dilations to upstream arteries. Further study of the cerebrovascular endothelium is critical for understanding the pathogenesis of a number of pathological conditions, including stroke, traumatic brain injury, and subarachnoid hemorrhage.

Responses of cerebral arterioles to ADP: eNOS-dependent and eNOS-independent mechanisms

ADP mediates platelet-induced relaxation of blood vessels and may function as an important intercellular signaling molecule in the brain. We used pharmacological and genetic approaches to examine mechanisms that mediate responses of cerebral arterioles to ADP, including the role of endothelial nitric oxide synthase (eNOS). We examined responses of cerebral arterioles (control diameter approximately 30 microm) in anesthetized wild-type (WT, eNOS+/+) and eNOS-deficient (eNOS-/-) mice using a cranial window. In WT mice, local application of ADP produced vasodilation that was not altered by indomethacin but was reduced by approximately 50% by NG-nitro-L-arginine (L-NNA) or 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) (inhibitors of NOS and soluble guanylate cyclase, respectively). In eNOS-/- mice, responses to ADP were largely preserved, and a significant component of the response was resistant to L-NNA (a finding similar to that in WT mice treated with L-NNA). In the absence of L-NNA, responses to ADP were markedly reduced by charybdotoxin plus apamin [inhibitors of Ca2+-dependent K+ channels and responses mediated by endothelium-derived hyperpolarizing factor (EDHF)] in both WT and eNOS-/- mice. Thus pharmacological and genetic evidence suggests that a significant portion of the response to ADP in cerebral microvessels is mediated by a mechanism independent of eNOS. The eNOS-independent mechanism is functional in the absence of inhibited eNOS and most likely is mediated by an EDHF.

THE ATP-SENSITIVE POTASSIUM-CHANNEL INHIBITOR GLIBENCLAMIDE IMPROVES OUTCOME IN AN OVINE MODEL OF HEMORRHAGIC SHOCK

This study was designed as a prospective laboratory experiment to evaluate the effects of the ATP-sensitive potassium-channel inhibitor glibenclamide on hemodynamics and end-organ function in an ovine model of hemorrhagic shock. Twenty-four adult sheep were anesthetized and surgically prepared to measure hemodynamics of the systemic and pulmonary circulation. The anterior surface of the abdominal aorta was exposed at a location 6 cm superior to the iliac bifurcation. After a 60-min period of stabilization, this location was punctured with a 14-G needle. To induce a hemorrhagic hypotension (mean arterial pressure [MAP] less than 50 mmHg) via bleeding, the needle was left in place for 15 s to insure good blood flow. Thereafter, it was removed, and the abdomen closed. The animals were then randomized to receive either glibenclamide (4 mg/kg over 15 min) or an equal volume of the vehicle, started 1 h postinjury. Hemodynamic variables were measured every 30 min. Compared with the control group, MAP and systemic vascular resistance index (SVRI) were significantly higher in the intervention group throughout the entire 6-h study period. Ileal pH and urine output were higher in treated than in control animals (4 h, ileal pH 7.29 +/- 0.31 vs. 7.17 +/- 0.6; 6 h, urine output 36 +/- 9 vs. 7.5 +/- 2 mL; P value less than 0.05 each). Because glibenclamide improved both hemodynamics and organ function, it may be a beneficial component in the acute treatment of hemorrhagic shock.

The role of the glia limitans in ADP-induced pial arteriolar relaxation in intact and ovariectomized female rats

We examined whether the glia limitans (GL) influences pial arteriolar relaxation elicited in vivo by the purinergic (P(2)Y(1) receptor) agonist ADP in female rats, and whether that influence is altered in ovariectomized (Ovx) females. A validated model for GL injury was used, topical application of the gliotoxin L-alpha-aminoadipic acid (L-alphaAAA), 24 h before the study. In both intact and Ovx females, L-alphaAAA had no effect on responses to the NO donor, S-nitroso-N-acetyl penicillamine, but ADP-induced pial arteriolar dilations were significantly reduced (by 33-90%), compared with vehicle-treated controls. When N(G)-nitro-L-arginine (L-NNA) was administered to L-alphaAAA-treated rats, the ADP response was virtually lost in intact females, but no further reductions were observed in the Ovx rats. On the other hand, in L-alphaAAA-treated Ovx females, when the gap junction blocker, Gap 27, was subsequently added to the suffusate, ADP reactivity fell to very low levels. In vehicle-treated control rats, L-NNA and Gap 27 reduced ADP reactivity by approximately 50% in intact and Ovx females, respectively. An earlier study indicated that the endothelium was a key site of influence for L-NNA (intact) and Gap 27 (Ovx). Thus present and previous results imply that the ADP response in pial arterioles represents the additive actions of an endothelial and a GL component. That supposition was confirmed in the present study by the finding that combining endothelial and GL injury produced an essentially complete loss of ADP reactivity in both intact and Ovx females. Finally, topical application of the selective P(2)Y(1) antagonist, MRS-2179, was associated with a nearly complete suppression of the ADP response in both intact and Ovx females. These results suggest that 1) ADP-induced pial arteriolar dilation involves additive contributions from P(2)Y(1) receptors present in both vascular endothelium and the GL; 2) the influence of the GL component is not altered by ovariectomy; and 3) the gap junction-dependent component of the ADP response in Ovx females is unlikely to include the GL and probably resides in the vessels themselves.

Nascent EDHF-mediated cerebral vasodilation in ovariectomized rats is not induced by eNOS dysfunction

In estrogen-depleted [i.e., ovariectomized (Ovx)] animals, an endothelium-derived hyperpolarizing factor (EDHF)-like mechanism may arise to, at least partially, replace endothelial nitric oxide (NO) synthase (eNOS)-derived NO in modulating cerebral arteriolar tone. Additional findings show that eNOS expression and function is restored in estrogen-treated Ovx female rats, while the nascent EDHF-like activity disappears. Because NO has been linked to repression of EDHF activity in the periphery, the current study was undertaken to examine whether the nascent EDHF role in cerebral vessels of Ovx females relates to a chronically repressed eNOS-derived NO-generating function. We compared the effects of chronic NOS inhibition with Nomega-nitro-L-arginine-methyl ester (L-NAME; 100 mg. kg-1. day-1 for 3 wk) on EDHF-mediated pial arteriolar vasodilation in anesthetized intact, Ovx, and 17beta-estradiol-treated (0.1 mg. kg-1. day-1 ip, 1 wk) Ovx (OVE) female rats as well as in male rats that were prepared with closed cranial windows. In the chronic NOS inhibition groups, pial arteriolar responses were monitored in the absence (all groups) and presence (females only) of indomethacin (Indo; 10 mg/kg iv). Finally, the gap junction inhibitory peptide Gap 27 (300 muM) was applied to block EDHF-related vasodilation. NO donor (S-nitroso-N-acetyl-penicillamine) responses were similar in all rats studied. Acetylcholine (ACh) reactivity was virtually absent in control Ovx rats and chronically NOS-inhibited intact female, OVE, and male rats. However, a partial recovery of ACh reactivity was seen in L-NAME-treated Ovx females. In addition, in the presence of L-NAME, a normal CO2 reactivity was observed in all females, whereas a 50% reduction in CO2 reactivity was seen in males. In intact and OVE rats, both chronic and acute (NG-nitro-L-arginine suffusion) NOS inhibition, combined with Indo, depressed ADP-induced dilation by > or =50%, and subsequent application of Gap 27 had no further effect on ADP-induced vasodilation. ADP reactivity was retained in Ovx rats after combined chronic NOS inhibition and acute Indo, but was attenuated significantly by Gap 27. In males, Gap 27 had no effect on arteriolar reactivity. Taken together, our data demonstrate that in the cerebral microcirculation, NO does not have an inhibitory effect on EDHF production or action. The increased EDHF-like function in chronic estrogen-depleted animals is not due to eNOS deficiency, suggesting a more direct effect of estrogen in modulating EDHF-mediated cerebral vasodilation.

Essential role of Gap junctions in NO- and prostanoid-independent relaxations evoked by acetylcholine in rabbit intracerebral arteries

BACKGROUND AND PURPOSE

Direct intercellular communication via gap junctions may play a central role in endothelium-dependent relaxations that are mediated by a conducted hyperpolarization and do not involve the synthesis of NO and prostanoids. In the present study, inhibitory peptides homologous to the Gap27 domain of the second extracellular loop of connexin37/connexin43 and connexin40, designated as 37,43Gap27 and 40Gap27, respectively, were used to evaluate the role of this mechanism in intracerebral arteries.

METHODS

Isolated rings of rabbit middle cerebral artery were constricted by histamine (10 micromol/L) in the presence of N(G)-nitro-L-arginine methyl ester (300 micromol/L) and indomethacin (10 micromol/L). Concentration-relaxation curves for acetylcholine were constructed in the presence and absence of 37,43Gap27 and 40Gap27. Specific antibodies were used to delineate the distribution of connexin37, connexin40, connexin43, and connexin45 within the arterial wall.

RESULTS

Individually, 37,43Gap27 and 40Gap27 minimally affected endothelium-dependent relaxations to acetylcholine at concentrations of 300 micro mol/L, whereas their combination (at 300 micromol/L each) inhibited the maximal response by approximately 70% and increased the EC50 value for relaxation by approximately 15-fold. In endothelium-denuded rings, this peptide combination did not attenuate responses to sodium nitroprusside, an exogenous source of NO. Gap junction plaques, whose incidence was highest in endothelium, were constructed from connexin40 and connexin43 in the media and connexin37, connexin40, and connexin43 in the endothelium.

CONCLUSIONS

The findings confirm that direct communication via gap junctions contributes to agonist-induced relaxations of intracerebral arteries. More than one connexin subtype appears to participate in such responses.

Role of cytoplasmic phospholipase A2 in endothelium-derived hyperpolarizing factor dilations of rat middle cerebral arteries

Very little is known regarding the mechanism of action for the endothelium-derived hyperpolarizing factor (EDHF) response in cerebral vessels. The authors tested two hypotheses: (1) activation of the cytoplasmic form of phospholipase A (cPLA ) is involved with EDHF-mediated dilations in rat middle cerebral arteries; and (2) activation of the cPLA involves an increase in endothelial Ca through activation of phospholipase C. Middle cerebral arteries were isolated from the rat, pressurized to 85 mm Hg, and luminally perfused. The EDHF response was elicited by luminal application of uridine triphosphate (UTP) after NO synthase and cyclooxygenase inhibition (10 mol/L -nitro-l-arginine methyl ester and 10 mol/L indomethacin, respectively). AACOCF and PACOCF, inhibitors of cPLA (Ca -sensitive) and Ca -insensitive PLA (iPLA ), dose dependently attenuated the EDHF response. A selective inhibitor for iPLA2, haloenol lactone suicide substrate, had no effect on the EDHF response. The EDHF response elicited by UTP was accompanied by an increase in endothelial Ca (144 to 468 nmol/L), and the EDHF dilation was attenuated with U73122, a phospholipase C inhibitor. The authors conclude that the EDHF response elicited by luminal UTP in rat middle cerebral arteries involved activation of phospholipase C, an increase in endothelial Ca, and activation of cPLA.

ADP-induced pial arteriolar dilation in ovariectomized rats involves gap junctional communication

It was previously shown that, despite the loss of nitric oxide (NO) dependence, ADP-induced pial arteriolar dilation was not attenuated in estrogen-depleted [i.e., ovariectomized (Ovx)] rats. Additional evidence suggested that the NO was replaced by an endothelium-dependent hyperpolarizing factor (EDHF)-like mechanism. To further characterize the nascent EDHF role in Ovx females, the current study was undertaken to test whether, in Ovx rats, ADP-induced pial arteriolar dilation retained its endothelial dependence and whether gap junctions are involved in that response. A closed cranial window and intravital microscopy system was used to monitor pial arteriolar diameter changes in anesthetized rats. The endothelial portion of the ADP-induced dilation was evaluated using light dye endothelial injury (L/D). The study was organized around three experimental approaches. First, the responses of pial arterioles to ADP before and after L/D exposure in intact and Ovx female rats were tested. L/D reduced the ADP response by 50-70% in both groups, thereby indicating that the endothelium dependence of ADP-induced vasodilation is not altered by chronic estrogen depletion. Second, the NO synthase inhibitor N(omega)-nitro-L-arginine (L-NNA) and the prostanoid synthesis inhibitor indomethacin (Indo) were coapplied. In intact females, L-NNA-Indo attenuated the response to ADP by 50%, with no further changes upon the addition of L/D. On the other hand, L-NNA-Indo did not affect ADP reactivity in Ovx rats, but subsequent L/D exposure reduced the ADP response by >50%. The NO-prostanoid-independent, but endothelium-dependent, nature of the response in Ovx females is a hallmark of EDHF participation. Third, gap junctional inhibition strategies were applied. A selective inhibitor of gap junctional function, Gap 27, did not affect ADP reactivity in intact females but reduced the the ADP response by 50% in Ovx females. A similar result was obtained following application of a connexin43 antisense oligonucleotide. These findings suggest that the nascent EDHF dependency of ADP-induced pial arteriolar dilation in Ovx females involves connexin43-related gap junctional communication.